Heterodyne circuits



Patented Apr. 9, 1935 UNITED STATES 1,997,012v HETERQDYNE CIRCUITSHarold 0. Peterson, Riverhead, N. Y., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application December 22, 1928, SerialNo. 327,802

5 Claims.

This invention relates to the combination of energies having differentfrequencies, and more particularly to heterodyne circuits.

It is frequently necessary to combine energies in order to obtainresulting energy of different frequency, particularly in modulationA andin heterodyning. In heterodyning, the frequencies involved are usuallyrelatively` closely adjacent, so that the tuned circuits for each of thefrequencies, unless reactively insulated in somefashion, are vapt toaffect one another during the tuning of one or the other. With shortWave energy this difculty is quite marked, even if the frequencies arenot close. It is aprimary object of my invention to prevent couplingbetween such circuits, so that they may be independently, tuned, andthis I do by coupling one of the cir,-y

cuits to the combining means through an untuned or detuned couplingtube.

When dealing with very high frequencies the capacitive coupling betweenthe electrodes of the coupling tube may become appreciable, and in suchcase I nd that this objection may be overcome by the use of ascreen-grid tube as a coupling tube, in order to improve the reactiveinsulation.

It may prove desirable, in some cases, to utilize a harmonic of thelocal oscillator frequency for combination With the received energy, andin such case the practice heretofore has been to letthe detector tubewhich is used to combine the received and locally generated energiesalso function as a harmonic generator for the locally generated energy.In accordance with my invention, for this purpose I merely bias thecou-l pling tube sufficiently to cause it to generate the desiredharmonic which is to be fed to the heterodyne detector.

My invention is described more in detail in the following specification,which is accompanied by a drawing in which Figure 1 is a Wiring diagramfor a heterodyne receiver employing my invention; and Figure 2 is amodification in which the receiver circuits are symmetrical pushpullcircuits.

Referring to Figure 1 it will be seen that energy is collected on anantenna circuit 2, and coupled through a coil 4, to the resonant inputcircuit 6 of an amplifier tube 3. y A grounded astatic shield I0 may beprovided between the coil 4 and the circuit Ii in order to preventcapacitive coupling therebetween. The anode of the tube 8 is connectedto a resonant circuit I2, which, like the circuit 5, is tuned to thefrequency of the desired signal. This resonant circuit I2 acts as anoutput circuit for the ampliiier tube 8, and also as an input circuitfor the heterodyne detector or combining tube I4, which is given anasymmetric characteristic by means of a blocking condenser I6, and asource of high negative bias potential I8. The energy with which thereceived energy is to be heterodyned is generated in a local oscillator20, the resonant output circuit 22 of which is tuned to the frequency ofthe locally generated energy. This circuit is coupled to the controlelectrode of the detector tube I4, preferably by a lead 24 connected tothe lower potential side of the resonant circuit I2.

Now, in accordance With my invention, the means connecting theoscillator 20 with the heterodyne detector includes an untuned couplingtube, here numberedV 26. As shown, the resonant cir-V cuit 22 islconnected to the control electrode of. the coupling tube 26, and theanode of the coupling tube is connected tothe lead 24 and thence 'to theresonant circuit I2 andthe control electrode of the detectorl tube I4.The tube 26, may be considered as untuned, or, including circuit I2, as,detuned. When dealing with very high frequencyenergythe tube 26, as Wellas the ampliiier tube 8, may preferably be of the screengrid type. Asvis customary, the screen-grid of the tube 26 is polarized atthe properpotential by a battery 28, but is connected directly to ground, withrespect to radio frequencies, by a, blocking condenser 30.

The anode of the detector tube I4 may include a high pass filter 32 forbyepassing the radio frequency components of the detected energy, andthe output, if an audio frequency, is translated in any suitabletranslating device 34, here shown coupled to the anode circuit of thetube I4 by a transformer 3e. It should be understood that in the case ofa superheterodyne receiver the output from the detector I4 may be anintermediate frequency, Which then is further amplified in anintermediate frequency amplier, before the nal detection andtranslation.

It win be noticed that the various stages of the apparatus are shieldedto prevent mutual in- 45 terference, and the necessity for thisshielding increases With the frequencies being handled.

If a harmonic of the local oscillator frequency is to be employed in theheterodyne detector the negative bias of the tube 26 is madesufficiently high to give the tube an asymmetric characteristic, inorder to provide harmonics in its output.

In order to provide symmetrical receiving circuits it is often desirableto employ pushpull aniplifier and detector stages. In such case thelocal to the pushpull input circuit of the heterodyne detector, whichprovides considerable insulation against mutual reaction between thetuning of the oscillator and the tuning of the receiver circuit.However, at very high frequencies I find that this alone is notsuiiiciently preventative, and that my invention may be applied to thistype of circuit with equally good results. An arrangement illusratingthis is shown in Figure 2, in which the antenna circuit 2 is coupledthrough a coil 4, provided with a grounded astatic shield I0, to theresonant input circuit 6 of a pushpull' amplifier stage using theamplifier tubes 52 and 54. The anodes of these tubes are connected inseries by the resonant circuit I2, which acts as a pushpull outputcircuit for the amplifier tubes 52 and 54, and as a pushpull inputcircuit for the heterodyne detector tubes 56 and 58. The anodes of thedetector tubes are connected in series to an output coil 69, which iscoupled to a translating devicev 34. Condensers 62 may be used toby-'pass the radio frequency components' of the detected energy. Y

The control electrodes of the tubes 52 and 54 are biased by a potentialsource 62, which is connected to the midpoint of the coil of the inputcircuit 6, While the detector tubes 56 and 58 are biased for detectoraction by a source 64, which is coupled to the midpoint of theresistance 66. The anodes of the tubes 52 and 54 are polarized from apotential source 68, which-is connectedy to the midpoint of the coil ofthe circuit l2,-and the anodes of the detector tubes 56y and 58 arepolarized by a source 10, connected to the midpoint of the output coil60. The oscillator20 is connected through a coupling tube-26 and a lead24 to the midpoint of the coil of the resonant circuit Vl2. This being azero potential point with respect to the received frequency, there is animmediate reduction in the tendency-for reactive coupling between thecircuit I2 and the oscillator circuit 22, and any remaining tendency iscounteracted by the reactive insulation between these circuits which isprovided by the coupling tube 26. l

In this case the tube 26 need not` be a screengrid tube, and preferablyisy not a screen-grid nary screen-grid tubes, for the reason 'that theAtube is coupled to the mid potential point of the coil of the circuitI2, and must feed that circuit with considerable energy. However, if alow impedance screen-grid tube is available it may well be used;

As before, the control electrode of the tube 26 is biased from asuitable source of biasing potential, `the magnitude of which determineswhether or not the coupling tube 26 is to act also as a harmonicgenerator.

In the claims which follow I shall use the term untuned to mean eitheruntuned or detuned, broadly, that is, an electron emission tube arrangedprimarily, not for amplification, but rather for reactive insulation.

I claim: v

l. In combination a source of radio frequency energy, shielding meansVfor said source, a second source of radio frequency energy, thefrequency of said last named source differing from the `frequency ofsaid first named source, shielding means for said last named source, athermionic device for combining energies from both of said sources,shielding means for said last named device, an aperiodic screen gridthermionic device for coupling one of said sources to said combiningdevice, and shielding means for said coupling device.

2. Means for demodulating a signal modulated high frequency carriercomprising, thermionic means responsive to said carrier, thermionicrectifying means coupled to said carrier responsive means, a thermionicoscillation generator, and means for coupling said thermionic responsivemeans and said thermionic oscillation generatorl to said rectifyingmeans and for preventing reaction between said means comprising aAscreen grid thermionic tube having its input electrodes coupled throughan untuned circuit to said generator and its output Aelectrodes coupledto said rectifier, and separate shielding means for said recti-V fyingmeans, for said generator, and for said coupling tube.

3. In combination, a signal source of energy, another source of energycomprising' an oscillation generator, a device for combining saidenergies, a path coupling said signal source to said device, and anotherpath including a screen grid l tube for coupling said oscillationgenerator to said device, and separate individual shielding means forsaid sources, said screen grid tube and said device.

4. In combination, in a heterodyne receiving circuit, an antenna, anelectron discharge device having input and output circuits tuned to theincoming frequency, said input circuit being coupled to said antenna,said output circuit including an inductance, a detector, an oscillationgenerator, a connection from one end of said inductance to saiddetector, and a connection from a point intermediate the ends of saidinductance to said oscillation generator, said last connection includingan untuned screen grid coupling tube for preventing reaction betwn saidoscillation generator and said output circuit'.

5. In combination, a signal lsource of energy, an oscillation generator,a circuit tuned to the frcquency of said signal source comprising aninductance, a device for combining the energies from said signal sourceand said oscillation generator, a connection from said signal source andfrom said device to the same terminal of said inductance, and aconnection including a screen grid tube from a point intermediate theends of said inductance to said oscillation generator.

HAROLD O. PETERSON.

